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Lexical precedence relation:    In traditional approaches, a prece-
dence ordering is placed on all terminal symbols T so that if the
regular expressions for more than one terminal match the same prefix
of the input string, the one with the highest precedence is selected.
In tools like Lex, this precedence ordering is a total order- ing,
implicitly specified by the textual order in which the terminals
appear in the lexical specification.
In our approach, we cannot base any precedence ordering for terminals
on their textual order in the grammar specification, as the extensions
that contain the terminal specifications are composed with the host
language as an un-ordered set, not an ordered list. Fur- thermore,
many terminals never appear in the same valid lookahead set and
thus do not need precedence orderings to disambiguate.

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